1. Field of the Invention
The present invention relates to a heat generating resistor, and more particularly to a heat generating resistor suitable for a recording head such as a liquid jet head which jets recording liquid by applying thermal energy to the recording liquid or a thermal head, a liquid jet recording head using such a heat generating resistor, and a drive method therefor.
2. Related Background Art
In a recording head such as a liquid jet recording head which jets recording liquid by applying thermal energy to the recording liquid by using a heat generating resistor or a thermal head which prints characters by applying thermal energy to a transfer ribbon or thermo-sensitive paper by using the heat generating resistor, it is important to lengthen the lifetime of the heat generating element. In part, damage to the heat generating resistor is, in many cases, due to nonuniform heat generation in the heat generating resistor which serves as a heater.
It has been proposed in a heat generating resistor having a conductive electrode layer formed on a heat generating resistive layer to widen the heat generating resistive layer on which the electrode is formed wide than a width of the electrode in order to prevent the electrode from being broken when the electrode is formed and increase step coverage of a protective layer to enhance durability. (See Japanese Patent Application Laid-Open No. 194589/1984). However, in the heat generating resistor of such a shape, a density of a current flowing across the electrodes is not uniform but concentrates to a certain point. As a result, the heat generation is not uniform but the heat generation is large at a certain area of the heat generating resistor. Damage arises from the large heat generation area and the lifetime of the resistor is shortened consequently.
In the present invention, a relationship between the width of the heat generating resistor and the width of the electrode is considered where the former is larger than the latter.
Problems encountered in the prior art will be explained in connection with a liquid jet recording head which uses a liquid jet recording method to jet liquid by utilizing a thermal energy.
A liquid jet recording method for the liquid jet recording head disclosed in DOLS 2843064 is characterized over other liquid jet recording methods in that it applies thermal energy to liquid to produce a motive force to discharge droplets. In the disclosed method, the liquid acted on by the thermal energy is overheated to generate bubbles, and the liquid is discharged from an orifice at an end of the recording head by an action of the bubble generation so that flying droplets are formed, and the droplets are deposited to a recording medium to record information.
The recording head used in this recording method usually comprises a liquid discharge unit having an orifice from which liquid is discharged and a liquid flow path including a heat action area which communicates with the orifice and by which thermal energy for discharging droplets act on the liquid, and a heat generating resistor or heat generation unit for generating the thermal energy.
A shape of the heat generating resistor as shown in FIG. 1 has been proposed. Requirements to define such a shape are as follows. It is defined by a ratio of a maximum value of a gradient of .phi., .sqroot.(.differential..phi./.differential.x).sup.2 +(.differential..phi./.differential.y).sup.2 to a value of .sqroot.(.differential..phi./.differential.x).sup.2 +(.differential..phi./.differential.y).sup.2 at a center of the resistor when a Laplace equation .differential..sup.2 .phi./.differential.x.sup.2 +.differential..sup.2 .phi./.differential.y.sup.2 =0 is solved for the heat generating resistor area when an orthogonal coordinate system X-Y is defined on a surface of the heat generating resistor 3, .phi.(x,y) is defined as a potential at a point (x,y) on the surface of the resistor, a certain boundary value is imparted to an area of a circumferential boundary of the resistor which contacts to one electrode 4, and a different boundary value is imparted to an area which contacts to the other electrode 4, a boundary condition in which a differential coefficient of .phi. to a normal direction of the circumferential boundary is zero is imparted to an area which does not contact to any of the electrodes.
For example, the ratio in the prior art resistor shown in FIG. 1 is mathematically infinite.
The above heat generating resistor has a pair of electrodes which are usually a selection electrode and a common electrode. A voltage is applied across the electrodes so that thermal energy for discharging droplets from the orifice is generated from the heat generating resistor. One of the major factors to determine a repetitive usage lifetime (durability) of the liquid jet recording head is a mechanical impact force called a cavitation destruction which is generated when vapor bubbles extinguish by self-contraction more specifically, the cavitation destruction occurs as the liquid near the heat generating resistor is overheated by abrupt heat generation by the heat generating resistor and it reaches an overheat limit temperature of the liquid and vapor bubbles are generated, and the liquid is discharged from the orifice by rapid volume increase and flying droplets are formed. As the bubbles (vapor bubbles) extinguish by self-contraction, the cavitation destruction occurs. The impact to the heat generating resistor by the cavitation destruction has been a factor to determine the durability of the recording head.
Several approaches to improve the durability of the recording head by avoiding the above problem have been known. For example, the heat generating resistor is made of a high anti-cavitation property, or a protection layer having the high anti-cavitation property is provided between the heat generating resistor and the recording liquid, or the liquid flow path is structured to weaken the impact force by the cavitation destruction. The durability of the recording head has been improved by those approaches.
In a dot print type liquid jet recording head which utilizes thermal energy and in which the heat generating resistor is laminated on a substrate of a liquid path which communicates with the orifice and the liquid is heated by supplying a pulse to the heat generating resistor, it is important for the improvement of image quality to effectively apply thermal energy to the liquid for each pulse and stably discharge the liquid when the head is repeatedly driven.
It has been known that the above object is attained by laminating on the substrate a lower layer having a thermal conductivity k.sub.2, a specific heat c.sub.2, a density .rho..sub.2 and a thickness L.sub.2, a heat generating resistor layer having a thermal conductivity k.sub.H and a thickness L.sub.H, and an upper layer having a thermal conductivity k.sub.1, a specific heat c.sub.1, a density .rho..sub.1 and a thickness L.sub.1, in this order with materials and dimension being selected to meet relationships of ##EQU1## where L=L.sub.1 +L.sub.H +L.sub.2 ##EQU2## .tau.: half-value width of an electrical signal applied to the heat generating resistor
t: time between input of one electrical signal and input of the next electrical signal PA1 S: area of thermal action surface on a surface of the upper layer facing the thermal action area PA1 .DELTA.T: mean value of differences between surface temperatures of the thermal action surface and temperatures of surface of the lower layer facing the substrate PA1 Q: heat generated by one electrical signal
In order to meet a requirement of higher durability, there still remains a problem even if the above formulas are met.